The present invention relates to a method and device for decreasing the working time necessary for floating offshore drilling for petroleum products and for increasing the safety of the work. The invention further relates to a support cart, from which can be hung a long string of tubulars, and a method of using the support cart to allow installation of the set of production valves known as a Xmas tree without the need to completely pull the blow-out preventor (BOP) out of the water.
Locating oil in deep water requires the use of large and expensive drilling rigs. These rigs can be drill ships, barges, tension leg platforms (TLPs), spars, or semi-submersibles. In each case, the investment is substantial. However, the need to find additional reservoirs of oil and natural gas is creating a need for a deep-water rig that can efficiently explore, drill and develop wells in ever deeper water, with current technology moving to achieve production at 7500 feet or greater.
The cost of developing a well is more than simply the cost of the rig. Every day of rig operation can cost in the hundreds of thousands of dollars. Therefore, it is important that operations run as smoothly, quickly, and safely as possible. One direction of development in offshore drilling is in multi-tasking jobs, that is, rather than performing the drilling and casing as one long set of sequential steps, some steps are effectively moved xe2x80x9cofflinexe2x80x9d, where they can be handled in parallel with other steps. For example, U.S. Pat. No. 6,056,071, which is hereby incorporated by reference, discloses an offshore drillship in which the single derrick has two top-drives (main and auxiliary) and two sets of equipment to handle tubulars (drill pipe, casing, etc.). While the main drive is handling a current step, the auxiliary drive can be preparing equipment for a subsequent step.
Once an offshore drilling rig is moored on the drill site, the general flow of operations is as follows: the drilling starts with the largest size drill bit to be used for this site, e.g. 36 inches. A string of drill pipe is built and extended to the sea floor. The well is drilled to a given depth with this size bit, then the drill string is pulled out and casing is placed in the wellbore and cemented in place. Then the next smaller size of drill bit is installed on the workstring, and the next section of wellbore is drilled, then cased, and so on. As soon as sufficient pipe has been cemented in the hole to provide a competent structural foundation for the seabed pressure containment equipment, a blowout protector (BOP) is run on marine riser pipe and placed at the sea bed on the well head and remains in place until all drilling and casing are complete. Marine riser normally consists of large diameter pipe sections each between 50 ft and 100 feet long and connected together with high strength bolts, dogs or other mechanism. Each section supports additional smaller diameter high-pressure pipes and electrical control cable and the assembly and disassembly of the marine riser string in deep water is a time consuming operation. Once in place, the riser provides an enclosed space through which the drill pipe and bit will be lowered to the borehole, and through which the drilling mud will be returned from the borehole to the drill rig. Once the well is drilled and cased, and production tubing is installed, the BOP must be removed from the wellhead and replaced with a production structure known as a Xmas tree. It is desirable to remove the BOP and install a Xmas tree in as short a time as possible, as the wellhead is not protected while the switch-over is happening. In installations where there is only one hoisting path, the BOP and marine riser must be pulled up and disassembled before the Xmas tree can be lowered on its string of tubing, requiring a great deal of time for the entire operation. In an installation having two hoisting paths, the Xmas tree is run on one load path while the BOP and riser is suspended on the other. The BOP and riser can be disassembled after use, or it may be desirable to immediately re-use them at an adjacent well site without disassembling them. Because the two load paths are in the same derrick and therefore close together there is a danger that the Xmas tree and the BOP will at some point collide and damage either one or both. It would be desirable to have a method of obtaining more separation between these two pieces of equipment during handling, without affecting the savings in time gained by the two load paths.
This invention provides a means to suspend the BOP and riser string in such a manner that it can then be moved to the other end of the moon pool from the load path on which the Xmas tree is run. The horizontal separation provided by this maneuver is sufficient to reduce the risk of collision between BOP and Xmas tree to an acceptable level.
To suspend and move the string, a cart is provided which runs on rails located on both sides of the moonpool area. These rails are also used by the BOP elevator, which supports and stabilizes the blowout protector while it is being moved from storage to its underwater site. The cart is of heavy construction, with a rectangular base and wheels at each corner. A funnel-shaped opening runs through the cart, narrowing from top to bottom, while a roughly C-shaped opening in the front of the cart is wide enough to allow a marine riser or other tubular to be admitted within the opening. The cart can be moved on rails to the load path that is handling the BOP, so that the riser and BOP can be suspended from the cart. The cart, with the riser and BOP hanging from it, can then be moved to one side of the moonpool, where it will remain until it is required again by the operation. In one embodiment of the invention, the blowout protector (BOP) is disconnected from the well head, raised some distance above the seabed (e.g., 50 feet), and hung from the cart. After the cart is moved to the side of the rig opposite the hoist handling the Xmas tree, the Xmas tree assembly can be installed, without the need to completely pull the BOP and riser out of the water. Once the Xmas tree assembly is in place, the BOP can be pulled up or moved to an adjacent well head. This increases the safety of this exchange without slowing the path of critical operations.